SDDs have tried to displace HDDs in computers for a few years but the higher cost of flash memory has been a major barrier to wide-spread adoption.  Lower flash memory prices will help adoption but HDDs decrease in $/GB at about the same rate as SSDs so the relative ratio of prices doesn’t improve much in SSDs’ favor.  At the same time there are serious issues in performance for many computers with HDDs, associated with the slower access time of HDDs. 

There have attempts to combine the advantages of HDDs and flasy memory in the past such as Intel’s Turbo Memory and the hybrid hard disk alliance but these were mostly  dependent upon the operating system to provide performance advantages.  The latest initiatives to combine flash memory and hard disk drives to create tiered storage systems in computers are known as Blink Boot, hyperHDD and the solid state hybrid hard drive.

Most of these approaches (hyperHDD and solid state hybrid hard drive) don’t depend upon the operating system to manage the use of the flash memory and the HDDs.  In the case of the recent solid state hybrid hard drive from Seagate the 4 GB of  flash memory  on the PCB board of the hard drive is used to store the most recently accessed data that the computer is using.  This is done internally by the hard drive providing a boost in access speed for this content without any special requirements on the computer operating system.

By adding a little flash memory to a hard disk drive for frequently accessed data or even for OS and application booting while still keeping the HDD for inexpensive mass storage makes a lot of sense.  Computer storage tiering with flash memory and HDDs could finally help flash memory become mainstream in computers.

Some SSD advocates project that SSD price per gigabyte will cross over that of HDDs, due to slower growth in areal density of HDDs in the future than it has grown in the past.  HDD price per GB declines will slow as a result of a slower areal density growth.  The argument is that this would allow flash gigabyte prices to blow past HDD prices just as they slipped below DRAM gigabyte prices in 2004.

 Some of these advocates recently predict that 3.5″ HDDs will “only” reach 6TB by 2015.  Although we find it likely that we will have 6 TB HDDs in mass production by then, Coughlin Associates expects to see 10TB maximum announced product capacity of 3.5-inch hard disk drives by that time. 

 Slower areal density growth of hard disk drives may result from transition difficulties to new recording technologies such as patterned media and heat assisted magnetic recording.  It appears likely that we may see areal density growth slowing from 40-50% annually today to 20% or possibly even less over the next few years.  Today the HDD industry is shipping 2 TB 3.5-inch HDDs and 1 TB 2.5-inch HDDs and will likely ship 3 TB or larger (3.5-inch) drives in the second half of 2010.  If the areal density of HDDs increased only 20% annually from 2010 through 2015 this would give us 7.5 TB 3.5-inch HDDs and over 3 TB 2.5-inch HDDs.

 Although a slow down is likely during a technology transition phase it is likely going to slow down gradually from today’s roughly 40% annual areal density growth rate.  So let’s say we have one more year of 40% growth (2010-2011), one year of 30% growth (2011-2012) and then 20% growth for the three remaining years to 2010.  With a 3 TB capacity in 2010 that would give us a 9.4TB capacity in 2010.  There is enough uncertainty in these numbers that the actual capacity could be between 8 and 11 TB so let’s say the maximum storage capacity in 2010 for 3.5-inch drives is 10 TB.  Likewise because of the geometry differences the maximum 2.5-inch storage capacity would be about 5 TB. 

 If the HDD industry stays true to its history, these 10TB HDDs will cost $50, giving a price per terabyte of $5.  Meanwhile, NAND flash terabyte prices will have declined to $50-100, preventing SSDs from displacing HDDs at least through 2015!